In practice the twenty-one are met as a set rather than one at a time. The praise liturgy runs through all of them in order, and a practitioner reciting it addresses each in turn without stopping on any one, so for most devotees most of the twenty-one exist as a verse and a colour rather than as a figure with a devotional life of her own.
Two are the exception, and they are the two the wider world knows. Green Tara, swift and active, is the form most often painted, cast and invoked, and the one ordinary devotional practice is built around. White Tara, associated with long life and healing, has her own extensive practice and her own place in Tibetan longevity ritual. Between them they account for the overwhelming majority of images an outsider will ever see, and a visitor to a Tibetan or Nepali shrine can pass a great many Taras without meeting any of the other nineteen individually.
The set itself is not fixed as tightly as the number suggests. More than one enumeration of the twenty-one circulates within Tibetan Buddhism, and they do not agree on the iconography of every form, so a painted set of twenty-one from one transmission will not match a set from another in every attribute. The number is stable, the verses are stable, and the visual identification of the individual forms is where the traditions diverge.
A stable liturgical set with an unstable iconographic one is why the twenty-one are usually treated together. The liturgy names them individually, but treating each as a separate deity would suggest an independence the practice does not give them.